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The massive earthquake that triggered the Asian tsunami wobbled the Earth on its axis, forced cartographers back to the drawing board and changed time by a fraction. But there's no need to adjust your clocks.

Six weeks after the tsunami that may have killed 300,000 people on the shores of the Indian Ocean, scientists are discovering more about the changes wrought by the magnitude 9 quake, the fourth-largest in the past century.

It caused upheaval on the sea floor near its epicentre off the northwest coast of the Indonesian island of Sumatra and moved several other islands.

But scientists say any movement of land mass can be measured in centimetres rather than tens of metres.

Dr Chen Ji, a seismologist at the California Institute of Technology, says he found movement along the fault line of about 10 metres laterally and four or five metres vertically.

But reports that the entire island of Sumatra, 1700 kilometres long and 400 kilometres wide, moved 35 metres or more are wildly inaccurate, scientists say.

"We know we have movements of over a metre, perhaps a couple of metres," says Dr Ken Hudnut, a California-based geophysicist with the US Geological Survey. "But the idea that Sumatra has moved [30 metres] is just wrong."

Scientists are working on precise measurements by comparing geographic points whose locations were known before the quake with their new positions using the global positioning system, which reads exact locations by satellite.

High-tech UK and US ships are investigating changes to the sea bed and local authorities are measuring depths in critical shipping channels.

Shorter day

NASA scientists say the 26 December quake, the largest to rattle Earth since 1964 in Alaska, disrupted the planet's rotation and shaved 2.68 microseconds, or millionths of a second, from the length of a day.

NASA scientists Dr Benjamin Fong Chao and Dr Richard Gross calculated it shifted Earth's mean north pole about 2.5 centimetres and made the planet slightly less oblate, or less flattened at the poles.

"Physically, this is analogous to a spinning skater drawing arms closer to the body, resulting in a faster spin," they write in an article in Eos, a publication of the American Geophysical Union.

But they say these changes are based on calculations rather than measurements. The changes are so small they are either difficult to measure or too small to detect.

Many earthquakes shake the planet's axis and affect its rotation, scientists add, but their impact is too small to measure.

But environmental damage from the tsunami was vast. The killer waves gouged beaches, crushed coral reefs, smashed thousands of hectares of mangrove forests and refashioned coastlines from Thailand to Somalia.

A preliminary survey by Indonesia's government and the United Nations Environmental Programme (UNEP) estimated the economic cost to the environment at US$675 million (A$882 million) in Indonesia alone.

The survey says 25,000 hectares of mangroves and 29,000 hectares of coral reefs were damaged.

Reefs, mangroves

Some coral reefs were crushed by the waves. Corals grow slowly, some only a few centimetres a year, so their recovery could take decades.

John Pernetta, a UNEP official in Bangkok, says the extent of damage to some of the coral reefs around Thailand was up to 80% in some places. Their recovery is uncertain.

Mangroves torn out by the waves will fare better, he says, as they leave behind roots and seeds that will help them regenerate.

"Long-term damage to mangroves by hurricanes or tsunamis doesn't really happen," Pernetta says. "After five to 10 years you don't even know anything has happened."

Vast stretches of Sumatra's west coast were turned brown by the tsunami as rice paddies and other vegetation were swamped by salt water. It could take two or three rainy seasons to wash the salt from the saturated land, experts say.